I believe United and US Airways have the same policy for lap infants for international flights: 10% of the lowest fare available on the day you book your ticket. You could also buy your infant a seat with miles to save cash. I believe it has to be in the same class you are in but you may be able to game it to get buy a coach ticket for the infant that you will not use? Someone more familiar with the rules might be able to comment on that.

The consensus on lap infants in first class is that there is no consensus. It seems to be a polarizing topic, with some thinking there is nothing wrong with it and it is a family's right to fly first with infants, while others find it to be the height of selfishness. I tend to fall into the first camp and don't mind if infants are in first class (especially considering modern headphone technology and gate-to-gate electronics allowances these days) as long as the parents aren't letting older babies/toddlers crawl around in the aisle unsupervised.